Shell hardened torsion bar and process of making same



July 15, 1958 A. L. BOEGEHOLD 2,843,374 SHELL HARDENED TORSION BAR AND PROCESS OF MAKING SAME Filed July 16, 1954 4' I I O I oArm/vr: may awn-e INVENTOR 4:4 33/0/13 4 .4" m aim 3 aura/seerea/v azvrze-j" Y wyd United States Patent SHELL HARDENED TORSION BAR AND PROCESS OF MAKING SAME Alfred L. Boegehold, Pontiac, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application July 16, 1954, Serial No. 443,786

Claims. (Cl. 267--57) This invention relates to shafts subjected to torsional stresses such as torsion bar springs and the like and is more particularly concerned with an improved shell hardened bar. or shaft and process of making the same.

Torsion bar springs for motor vehicles are frequently made from a through hardening grade of spring steel such as SAE 9262, SAE 8660, SAE 5160, etc., having suflicient hardenability to yield an as-quenched hardness at the center of the bar of about 55 Rockwell C minimum. Following hardening, such torsion bars are tempered to a range of about 45 to 51 Rockell C. Shell hardening is also done on axle shafts. For this purpose a medium carbon steel such as SAE 1046 has been employed. Morgan Patent 2,599,575 describes such a shaft.

While the prior art procedures have produced shafts that are satisfactory in many instances, the prior art procedures have not produced shafts having as high and uniform a fatigue life as desired. Accordingly the primary object of the present invention is to provide an improved shell hardened torsion bar or shaft having a high and uniform fatigue life. Other objects and advantages of the invention will become more apparent as the description proceeds.

In accordance with the present invention there is provided a high carbon steel shaft of high carbon content with hardenability controlled, especially by manganese content, so that the ratio of core area to shell is such that the core will support the higher residual stress induced by the high carbon, high yield strength shell. In carrying out the invention a high carbon steel shaft consisting of .72 to .95 carbon, .30 to .60% manganese, silicon less than .50%, 040% phosphorus (main), sulphur .050% (max), and the blance iron, is heated to austenitize the same and is then rapidly cooled whereby a hard shell portion is formed at and adjacent the surface and the center or core portion is left considerably softer than the shell. It is to be understood that full hardening is accompanied by a volume increase greater than occurs in the partially hardened core thus in attempting to expand, the outer shell is opposed by the core material thereby setting up compressive stress in the case and tensile stress in the core. At the same time there is a sharp drop, in hardness between the shell and the core. That is, the drop in hardness from the hard shell portion to the softer core takes place in a relatively small distance inwardly from the hard shell portion thus producing an abrupt change from compressive stress in the case to tensile stress in the core. An austenitizing treatment that has been found especially advantageous is to heat the shaft to 1500 F.1550 F. for a time of minutes in a neutral salt bath. Following this, the shaft is rapidly pered shell having a uniform hardness within the range of about 55 to 60Rockwell C and a core having a hardness within the range of about 38 to 47 Rockwell C. The desired hardness of the shell portion and the desired hardness gradient below the shell portion are ob.- tained in extremely close limits by employing a high carbon steel shaft of the analysis given above. The high carbon content also. results in a relatively high hardness level in the core portion of the shaft. The higher core hardness is synonymous with higher strength than obtained in 1046 steel, cosequently a core having this higher hardness is able to maintain a higher level of compressive stress in the shell portion. than is possible when the core is relatively soft such as, for example,

20 to 35 Rockwell C which is the result. of using the lower r suificient 'unhardened core cross-section to assume the tensile stress for holding the shell or case in compresslon.

Reference is herewith made to the accompanying drawings in which:

Figure 1 is a torsion bar spring for use in a motor vehicle suspension system;

Figure 2 is a chart illustrating crossssectional hardness limits of a high carbon steel torsion bar spring or shaft of the present invention of 1 diameter austenitized and quenched in caustic soda solution. Included in this chart are the cross-sectional hardness limits for austenitized 1. diameter shafts as presented in the Morgan patent.

Figure 3 is a chart illustrating cross-sectional hardness limits of a high carbon steel torsion bar of the present invention of 1%" diameter austenitized and quenched in caustic soda solution. Included in the chart of Figure. 3 are the cross-sectional hardness limits of SAE 1045 steel austenitized and quenched in caustic soda solution.

In the drawings 10 represents a torsion bar spring having fixed to one end thereof a pad 12 for attachment to the frame of a motor vehicle or the like and a similar pad 12 at the other end for connection with an unsprung member.

As indicated by the solid line in the chart of Figure 2, a torsion bar spring of the present invention of a diameter of 1 has in the as-quenched condition a shell hardness of about 62 to 67 Rockwell C for a relatively shallow depth. The. hardness drops oif very rapidly leaving, a relatively thick core portion having a Rockwell C hardness of about 38 to 45. Illustrated by the broken. line in Figure 2 is a curve showing the hardness range and gradient cited by Morgan as obtainable with plain carbon steels of about 0.38 to 0.50% carbon. As indicated the hardness at the surface or shell of the shaft as described in the Morgan patent is considerably less than that of the steel shaft of this invention. The hardness of the Morgan material in Figure 2 does not drop as sharply from surface to center as does the torsion bar spring processed in accordance with the present invention. The hardness of the core also varies widely with the Morgan. material,

tion a shell hardness of about 62 to 67 Rockwell C for a relatively shallow depth. The hardness drops off rapidly leaving a relatively thick core portion having a Rockwell C hardness of about 40 to 47. Illustrated by the broken line in Figure 3 is a curve showing the approximate hardness range and gradient obtainable with a plain carbon steel of 0.380.50% carbon, as specified in the Morgan patent. It should be noted that this curve does not represent the same material as that shown by the broken curve of Figure 2, since translation of the Morgan data of Figure 2 from the 1 diameter shaft to the 1%" diameter shaft of Figure 3 results in a nearly uniform hardness from surface to center of the shaft. From this, it is concluded that the material on which Morgans curve in Figure 2 is based, is a 1045 or 1046 material modified with a hardenability intensifying agent.

A series of fatigue tests conducted on torsion bars or shafts in accordance with the invention over a stress range of 39,000 to 100,000 pounds per square inch showed no failure after 700,000 cycles. In another test a similar bar was run for 1,000,000 cycles without failure. On the other hand, tests made under the same conditions on torsion bars of SAE 9262 and 5160 steels, through hardened and tempered to 47 to 51 Rockwell C, showed average lives of only 325,000 and 123,000 cycles, re spectively.

Various changes and modifications of the embodiments of the invention described herein may be made by those skilled in the art without departing from the principles and spirit of the invention.

I claim:

1. A torsion bar spring comprising a high carbon steel shaft of circular cross section consisting of .72 to .95 carbon, .30 to .60% manganese, silicon less than .50%, .040% phosphorus (max.), sulphur 050% (max.), and the balance iron, and having a hard surface shell of relatively shallow depth and a softer core portion, the drop in hardness from the hard shell to the core taking place in a relatively short distance inwardly from the hard shell.

2. A torsion bar spring comprising an elongated high carbon steel shaft of substantially circular cross section consisting of .72 to .95% carbon, .30 to 60% manganese, silicon less than .50%, .040% phosphorus (max.), sulphur 050% (max.), and the balance iron, and having a surface shell of relatively shallow depth having a hardness within the range of about 55 to 58 Rockwell C and a softer core portion having a hardness within the range of about 38 to 47 Rockwell C, the drop in hardness from the hard shell to the core taking place in a relatively short distance inwardly from the hard shell.

3. The method of forming a shell hardened torsion bar spring which comprises heating to an austenitizing temperature a high carbon steel shaft of circular cross section consisting of .72 to .95% carbon, .30 to .60% manganese, silicon less than .50%, .040% phosphorus (max.), sulphur .050% (max.), and the balance iron, and rapidly cooling said austenitized steel shaft.

4. The method of forming a shell hardened torsion bar spring which comprises heating to an austenitizing temperature a high carbon steel shaft of circular cross section consisting of .72 to .95% carbon, .30 to .60% manganese, silicon less than .50%, 040% phosphorus (max.), sulphur .050% (max.), and the balance iron, rapidly cooling said austenitized steel shaft in a caustic soda solution and then reheating said shaft to a temperature of about 475 to 500 F.

5. A spring suspension system for motor vehicles comprising a torsion bar spring as in claim 1 and having means adjacent the end of the spring for connection to the sprung and unsprung members of the system.

6. A torsion bar spring for motor vehicles comprising an elongated carbon steel shaft having a diameter of at least approximately 1 /8 consisting of .72 to .95 carbon, .30 to .60% manganese, silicon less than .50%, .040% phosphorus (max.), sulphur .050% (max.) and the balance iron, and having a surface shell of relatively shallow depth having a hardness within the range of about 55 to 58 Rockwell-C and a softer core portion having a hardness within the range of about 38 to 47 Rockwell C, the drop in handness from the hard shell to the softer core taking place in a relatively short distance inwardly from the hard shell, and an attaching pad fixed to each end of the shell hardened shaft.

7. A torsion bar spring for motor vehicles compris ing an elongated carbon steel shaft having a diameter greater than one inch consisting of 0.72% to 0.95% carbon, 0.30% to 0.60% manganese, silicon less than 0.50%, 0.04% phosphorus (max.), sulfur 0.050% (max.), and the balance iron, and having a surface shell not over about inch in thickness having a hardness within the range of about 55 to 58 Rockwell C and a softer core portion having a hardness within the range of about 38 to 47 Rockwell C, said softer core portion having a diameter at least about /2 inch, and an attaching pad fixed to each end of the shell-hardened shaft.

8. The method of forming a shell-hardened torsion spring which comprises heating to an austenitizing temperature a high-carbon steel shaft having a diameter greater than one inch consisting of 0.72% to 0.95% carbon, 0.30% to 0.60% manganese, silicon less than 0.50%, 0.04% phosphorus (max.), sulfur 0.050% (max.), and the balance iron, and rapidly cooling said austenitized steel shaft to thereby form a hardened surface shell having a thickness not over about 7 inch having a hardness within the range of about 62 to 67 Rockwell C and a softer core portion having a hardness of about 38 to 45 Rockwell C, said core portion having a diameter of at least about /2 inch.

9. A torsion bar spring comprising an elongated carbon steel shaft of circular cross section having a diameter greater than one inch and a high carbon content of 0.72% to 0.95%, said steel having its hardenability controlled by a manganese content 'of 0.30% to 0.60% whereby after rapidly cooling from an austenitizing temperature a hardened shell is formed and a softer core portion, the ratio of area of core to shell being such that the core will support the high residual stress load induced by the high-carbon, high-yield-strength shell.

10. A torsion bar spring comprising an elongated carbon steel shaft of substantially circular cross section having a carbon content of 0.72% to 0.95%, said steel having its hardenability controlled by a manganese content within the range of 0.30% to 0.60% whereby quenching from an austenitizing temperature forms a hardened shell and a softer core portion, the ratio of area of core to hardened shell being such that the core will support the high residual stress load induced by the high-carbon, highyield-strength shell.

References Cited in the file of this patent UNITED STATES PATENTS 2,182,805 Hagenbuch er al. Dec. 12, 1939 2,508,130 Wharam et al May 16, 1950 2,599,575 Morgan June 10, 1952 OTHER REFERENCES Laminated Springs, by T. H. Sanders, The Locomotive Publishing Co., Ltd, 3 Amen Corner, London, E. C. 4, page 242. 

1. A TORSION BAR SPRING COMPRISING A HIGH CARBON STEEL SHAFT OF CIRCULAR CROSS SECTION CONSISTING OF .72 TO .95% CARBON, .30 TO .60% MANGANESE, SILICON LESS THAN .50%, .040% PHOSPHORUS (MAX.), SULPHUR .050% (MAX.), SAID THE BALANCE IRON, AND HAVING A HARD SURFACE SHELL OF RELATIVELY SHALLOW DEPTH AND A SOFTER CORE PORTION, THE DROP IN HARDNESS FROM THE HARD SHELL TO THE CORE TAKING PLACE IN A RELATIVELY SHORT DISTANCE INWARDLY FROM THE HARD SHELL. 